The present invention relates to a paper coating composition which provides coated paper having excellent water resistance, ink receptivity and excellent blister resistance, the coated paper being free from coloring and releasing only an extremely slight quantity of formaldehyde, and to a process for producing a novel thermosetting resin aqueous solution to be used as one component of the paper coating composition.
It has heretofore been known to produce coated paper having excellent printability by coating on paper a paper coating composition containing a pigment and an aqueous binder as major components and further containing auxiliary agents such as water resistance-imparting agents.
Recent years have seen remarkable progress in coated paper-producing techniques. For example, solids concentration of a coating composition is proposed to realize high-speed coating using a blade coater for the purpose of increasing productivity. Thus, it has been required to increase the solids concentration without an increase in viscosity for obtaining good coating properties.
In addition, with up-grading and speedup of printing and with rotary offset printing, a coated layer is required to possess higher water resistance, higher ink receptivity, and higher blister resistance.
In order to improve these properties, many proposals have been made, such as improvement of an aqueous binder component and improvement of auxiliary agents. For example, in order to improve water resistance, a water resistance-imparting agent is ordinarily compounded as an auxiliary agent because, though water resistance may be improved to some extent by increasing the weight ratio of aqueous binder to pigment, it in turn reduces ink receptivity upon printing coated paper.
Typical examples of the water resistance-imparting agents are those for aqueous binders. However, though conventionally used or proposed water resistance-imparting agents have some merits, they simultaneously possess serious defects, thus being practically unsatisfactory. For example, some problems with a melamine-formaldehyde precondensate are that it gives insufficient ink receptivity, that when the pH of the coating composition containing the precondensate is high, it shows insufficient water resistance, and that free formaldehyde is released from a coated paper in a significant amount. A reaction product between polyamidopolyurea and formaldehyde has not yet been provided having sufficient water-resisting effect and ink receptivity-improving effect, though such product shows some ink receptivity-improving effect, some water resistance, and releases free formaldehyde only in a slight amount from a coated paper.
Dialdehydes such as glyoxal are not preferable, because they color coated paper and show poor ink receptivity-improving effect. Multivalent metal salts such as zirconium salts are not preferable, either, because they seriously thicken the coating composition.